Tip #16: New Developments in Transmission Line Loudspeaker Design
By Dick Olsher (March 2000)
The Transmission Line loudspeaker (TL) was popularized by Bailey in the mid 60s and has to this day retained a cult following. Its origins date back to the Acoustic Labyrinth patented by Olney in 1936. Bailey's important contribution was to substitute fibrous line stuffing for the absorptive lining originally specified by Olney.
The idea of hanging a damped pipe onto the backside of a woofer has undergone much experimentation. Empirical guidelines have been developed regarding the optimum driver, pipe length and shape, and stuffing density. These guidelines have been debated over the past 45 years, as they often were found to poorly predict actual performance. No one had successfully forged a TL theory along the lines of Thiele-Small - that is until George Augspurger unveiled his work in September 1999. In two landmark papers presented at the 107th AES Convention Augspurger describes a computer analog model for TL systems, presents validation data derived from testing a variety of designs, and presents basic performance relationships similar to Thiele-Small theory. The papers are titled "Loudspeakers on Damped Pipes – Part One: Modeling and Testing" and "Loudspeakers on Damped Pipes – Part Two: Behavior" (preprint 5011). They are available for sale and downloading online at: http://www.aes.org/publications/preprints/. If you have any interest at all in TL design, these papers are must reading.
Augspurger provides optimized alignments (including required stuffing) based on Thiele-Small parameters for three TL geometries: tapered pipe, pipe with coupling chamber, and offset speaker. The woofer's VAS determines the required pipe volume while its free-air resonance frequency and pipe length determine the bass extension and overall Q of the system. In general, the bass half-power frequency (f3) for these designs can't be any lower than 80% of the nominal quarter-wave pipe resonance frequency (fp). The driver's free-air resonance frequency must be lower than fp. by as much as a factor of two for some of these alignments. The bottom line is that Augspurger's tabulated alignments make it easy to optimize TL performance for a particular woofer with known Thiele-Small parameters.